Though there have been many advances in prosthetic technology, existing systems are significantly limited in their ability to fully restore function after limb loss. Such limitations are manifested in the types of activities that can be achieved, the ease with which the tasks can be performed, and the richness of the experience. Improvements in state-of-the-art prosthetic technology can be achieved through fundamental advances in mechatronic systems, actuators, sensors, and control algorithms. However, truly advanced prosthetic systems require seamless integration of the intact sensory-motor living system with advanced, highly-capable artificial limbs, making it necessary to record neural activity to capture the motor-intent of the amputee.
It has been shown that despite partial atrophy and degeneration, both central and peripheral motor and somatosensory pathways retain significant function for many years following amputation—potentially allowing the use of neuroprosthetic technologies to establish efferent neural control of a prosthetic limb with direct afferent neural sensory feedback (Dhillon, Lawrence et al. 2004; Dhillon and Horch 2005; Dhillon, Kruger et al. 2005). Multiple electrode technologies currently exist to establish a neural interface. Choice of the electrode depends on many factors, including biocompatibility, long-term stability, ease of implantation, mechanical characteristics, electrochemical characteristics, and economics. Stimulation and recording can both be performed by electrodes placed either on the surface (surface electrodes) or beneath the skin (subcutaneous electrodes). Furthermore, subcutaneous electrodes can be placed on or in the muscle (epimysial or intramuscular electrodes), and on, in, or adjacent to peripheral nerves (extraneural or intraneural electrodes) (Keith 2001; Venkatasubramanian, Jung et al. 2006).
The current, most advanced hand and/or upper limb prostheses offer limited volitional motor control (Ohnishi, Weir et al., 2007). The primary pathway for the transmission of motor control information between the brain and muscle is through the peripheral nerves. Across numerous technologies, volitional motor control can be provided to a user at any level of the motor control pathway, but all of them require the recording of physiological signals. One approach is to record activity from peripheral nerves in the residual limb using longitudinal intrafascicular electrodes (LIFEs) and decode the signals to infer motor intent for the control of prostheses (Dhillon, Lawrence et al., 2004).
The few studies in which extracellular neural recordings from motor fascicles have been done in amputees using LIFEs indicate that the intrafascicular signal in the severed nerves has an ultra-low amplitude and broad shape. For example, when an amputee is attempting to generate activity of the missing limb, temporal 20 μV peak-to-peak signals can be obtained using intrafascicular recording (Dhillon, Lawrence et al., 2004). The signal-to-noise ratio is low, even with use of commercial recording systems (Dhillon, Lawrence et al., 2004; Lefurge, Goodall et al., 1991). Integrated circuits can be used for recording peripheral nerve activity (Uranga et al. 2004; Limunson et al. 2009; Rieger 2011). Often these use external components and are not always power/area efficient (200 μW/channel—order power and area above 0.06 mm2 per channel). FIG. 7 presents a typical recording circuit where the first active circuit is an amplifier.